Codex

A codex (the plural is codices) is the name bibliographers give to the format of
virtually all modern books: a series of sheets folded and bound
together so as to produce separate pages. The codex first
appeared around the fourth century C.E.,
and offered several advantages over the scroll, not least that it
was possible to jump easily from one part of a book to another. A
scroll pretty much has to be read sequentially: if you want to
get from the beginning to the end, you have to unroll the whole
thing. A codex, however, lets you turn from any one page to any
other with minimal problems. It made possible things like page
numbers and indexes, and helped to produce a revolution in the
way the written word was presented.

Note that the codex book long preceded the invention of printing.
For more than the first thousand years of its history, codices
were used for manuscripts, not printed books.